my chickens have breathing problems

2 of my 16 week old pure bred chickens that I have raised from chicks are having these breathig difficulties . I have had this same thing before and lost about 8 babies {3week olds} what is it ????, it is so frustrating watching them die the little ones died 1 by 1 everyday I would see another one start putting its head up and tryng to get air in. And every morning sure enough another was dead .. That is why I think whatever it is it can be passed around ....How can I stop it ...if it is a fungal infection does any one no of treatments. Any comments are appreciated as I have only been raising chickens for a couple of years so I am learning thanks Kinsburn